There is a great need for high accuracy height measuring gages for a wide variety of applications. One such application is in determining the position of a reticle in an electron beam lithography apparatus.
Electron beam lithography is rapidly becoming the method of choice for exposing ultrahigh accuracy reticles in the production of very large scale integrated circuits. An electron beam lithography apparatus typically includes an electron beam optics housing positioned over a vacuum chamber in which a reticle is installed. The reticle is a glass plate covered by a layer of chromium, with a layer of electron beam resist deposited over the layer of chromium. The reticle is mounted on a stage which moves the reticle in the X and Y directions under the control of the control system while the electron beam writes on, or exposes, the beam resist layer to produce the desired circuit pattern on the reticle. The control system not only moves the reticle to the desired X,Y coordinate for the stage position being exposed, but also, controls the beam deflection angle to control the point on the reticle at which the electron beam strikes.
In the past one of the more difficult problems in this art has been to determine the precise position of the reticle with respect to the electron beam optics. The precise position of the reticle must be determined in order to properly deflect the electron beam in order to accurately write on the reticle. It is extremely important that any system used to determine the position of the reticle be a vacuum compatible, compact, and noncontacting.